(1) Field of the Invention
The present invention relates to an AV data transmission apparatus, an AV data reception apparatus, and an AV data transmission and reception system, and particularly to an AV data transmission apparatus, an AV data reception apparatus, and an AV data transmission and reception system for encrypting AV data added with usage control information and transmitting the encrypted data.
(2) Description of the Related Art
In recent years, a home network that connects home appliances for sharing various contents is on the way to its implementation. Meanwhile, when handling data which is added with usage control information and requires copyright protection, such as newly released movies, pay-TV programs, and music, it is necessary to provide a processing unit that protects copyrights. An effective method for protecting copyrights is to encrypt data that requires copyright protection and to transmit the encrypted data, as well as protected usage control information of the data. This way, unauthorized use of the data is prevented.
A Digital Transmission Content Protection (DTCP) specification is an example of such a copyright protection specification in practical use.
In the DTCP specification, an AV data transmission apparatus and an AV data reception apparatus perform authentication and key exchange by an Authentication and Key Exchange (AKE) command so as to share an exchange key (Kx). The AV data transmission apparatus generates a content key (Kc) by using the exchange key (Kx), an encryption mode (EMI), and a key seed (Nc), and uses the content key as an encryption key. The AV data transmission apparatus encrypts AV data using the encryption key generated, and transmits the encrypted AV data.
Further, the AV data transmission apparatus stores the encryption mode (EMI) and the least significant bit of the key seed (Nc) in the header of a packet in which AV data is transmitted, and transmits the packet to the AV data reception apparatus. The key seed (Nc) itself is transmitted to the AV data reception apparatus by the AKE command.
The AV data reception apparatus generates a content key (Kc) by using the Kx shared through the authentication and key exchange, and the EMI and the Nc received from the AV data transmission apparatus, and uses the content key as a decryption key. The AV data reception apparatus performs decryption using the decryption key generated.
Further, it is necessary that the DTCP specification covers a cipher scheme in which the baseline cipher is specified as a cipher scheme. The DTCP specification covers an extended cipher scheme (optional cipher) for future extension. For future compatibility with the extended cipher scheme, cipher type information (Cipher_Algorithm) for selecting a cipher scheme is transmitted by an AKE command.
Furthermore, the DTCP specification defines “Copy Free”, “Copy One Generation”, “No More Copies”, and “Copy Never”, as copy control information which is usage control information for copy generation management of AV data. The AV data transmission apparatus transmits usage control information in association with an encryption mode (EMI), and the AV data reception apparatus manages, as shown in FIG. 9, for example, data use (copy control) based on the encryption mode (EMI). FIG. 9 is a table showing correspondence between EMI values and copy control information according to the DTCP specification.
Further, information such as Image Constrain Token for restricting resolution is defined as usage control information which is followed when AV data received by the AV data reception apparatus is outputted as analog signals. The usage control information is stored in a DTCP descriptor (DTCP_descriptor) and embedded in the AV data so as to be transmitted (Non-Patent Reference 1: Digital Transmission Content Protection Specification Revision 1.51 (Informational Version)).
Further, with copyright protection specifications related to AV data transmission, when a new network distribution specification or a new recording medium is introduced, new usage control information is sometimes defined to be added to AV data for transmission. To transmit AV data added with new usage control information, however, it is necessary for apparatuses to be capable of transmitting the new usage control information as well as AV data. Here, new usage control information refers to an extended version of conventionally-defined usage control information.
FIG. 10 illustrates transmission of various data between an AV data transmission apparatus that supports new usage control information and AV data reception apparatuses one of which supports new usage control information and the other of which does not support new usage control information. It also illustrates operations of these apparatuses. In FIG. 10, an AV data transmission apparatus 101 and an AV data reception apparatus 102 support new usage control information indicating “analog output prohibited”, whereas an AV data reception apparatus 103 does not support the new usage control information indicating “analog output prohibited”. FIG. 10 illustrates an example of an operating condition when various data is transmitted between the AV data transmission apparatus 101 and the AV data reception apparatuses 102 and 103.
With the DTCP specification, two methods are conventionally supported for transmitting usage control information, namely, a method of transmission using a DTCP descriptor and a method of transmission using an EMI. Therefore, it is expected that new usage control information will be transmitted by these methods.
As described, according to the transmission method using a DTCP descriptor, the AV data reception apparatus 102 supports new usage control information indicating “analog output prohibited”, and is thus capable of stopping analog output. The AV data reception apparatus 103, however, cannot recognize the new usage control information indicating “analog output prohibited”, and performs analog output without regard to the new usage control information. Thus, the new usage control information cannot achieve analog output control in the AV data reception apparatus 103.
As for the method using an EMI, since copy control information is already assigned to the four possible values to be taken by an EMI as shown in FIG. 9, it is necessary to add a new bit for assigning new usage control information indicating, for example, “analog output prohibited”. In that case, the AV data reception apparatus 103 does not support the added new bit (extended EMI), and thus cannot decrypt AV data. In addition, the AV data reception apparatus 103 cannot recognize the meaning of the new bit and recognize that the reason for not being able to decrypt the AV data is that the AV data was encrypted by the AV data transmission apparatus 101 using the extended EMI. As a result, the user of the AV data reception apparatus 103 could falsely recognize the situation as a failure of the AV data transmission apparatus 101 or the AV data reception apparatus 103, a failure of the network apparatus, or a transmission error.